Low cost high efficiency signal interrogation for multi-channel optical coherence tomography

ABSTRACT

A signal interrogation system comprises an optical coupler to split input laser beam into a first laser beam as a power reference and a second laser beam, the optical coupler being coupled to a first path for the first laser beam and a second path for the second laser beam; an optical circulator disposed in the second path; a bi-directional optical switch disposed in the second path and having on one side a single channel end oriented toward the optical circulator and on another side multiple channel ends with multiple switchable channels; a plurality of optical fibers coupled to the multiple channel ends of the bi-directional optical switch; an interference optical signals path coupled to the optical circulator to receive the interference optical signals from the bi-directional switch; and a balanced photo detector to measure a power difference between the interference optical signals and the power reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to imaging and, morespecifically, to a low cost high efficiency signal interrogationtechnique for multi-channel optical coherence tomography.

When lights reflected from samples interfere with a reference beam, thefrequencies of the interfering signals reveal the depth where the lightis reflected. This technique has been used in imaging, known as OpticalCoherence Tomography (OCT). OCT is an optical signal acquisition andprocessing method allowing extremely high-quality,micrometer-resolution, three-dimensional images from within opticalscattering media (e.g., biological tissue) to be obtained. In contrastto other optical methods, OCT, an interferometric technique typicallyemploying near-infrared lights, is able to penetrate significantlydeeper into the scattering medium, for example about three times deeperthan confocal microscopy.

The first generation OCT is a time domain technique that uses a widebandlight source and a time delay scanner. Only when the optical paths ofthe reflection lights and the reference beam are matched, they caninterfere and be detected. The significant drawback of this technique isa low imaging speed, which is limited by the speed of the delay linescanner.

In order to improve the imaging speed, a second type of OCT calledSpectral Domain OCT (SD-OCT) has been developed. Similar to the timedomain OCT, this technique also uses a broadband light source. Insteadof the time delay scanner, a transmission grating and a CCD array areused to interrogate the interfering signals. Since the speed for CCDarray scanning can be very high, this technique can be used for veryhigh speed 3D imaging. The disadvantages, however, are the heightenedcosts, the limited imaging depth and resolution.

Fourier domain OCT (FD-OCT) that uses wavelength-swept fiber lasersources is the third type of OCT. The coherence length resulting fromthe narrow instantaneous laser linewidth enables imaging up to 4 mmdepth in tissue. The wavelength sweeping rate has reached 100 kHz, whichis fast enough for 3D imaging in many applications. Of the three typicalOCT techniques described above, FD-OCT that uses wavelength-swept lightsources is the most suitable for commercial purposes in biomedicalimaging; this technique is cost effective, and has a faster imaging rateas well as improved resolution and sensitivity.

When a wavelength-swept light source and a fiber probe are used in OCTimaging, a 1D depth image, or A-scan, is obtained when the laser sourcemakes a complete scan. When the fiber probe is scanned across an object,a series of A-scans produce a 2D cross-section image, or B-scan. When aseries of 2D section imaging are accumulated, a 3D image is obtained.

For some applications, however, the process of scanning may beinconvenient or not economical, such as in catheter imaging. Probearrays could be used instead of scanning to form a multi-channel OCT.The prototyped 5-channel OCT has been developed by Thorlabs, using anoptical switch, with five photo detectors and fiber circulators, tomeasure five channel signals, respectively, as shown in FIG. 1. For a16-channel OCT, the cost for the signal interrogation would be over$30,000 at the time of drafting this patent application. Such a highcost renders it unsuitable for commercial purposes. Apart from the highcost, the cross-talking, and the bulk size, the most critical drawbackis that this interrogation technique is not suitable for a balanceddetection to remove the strong low frequency signal background, whichseriously reduces the measurement sensitivity.

Multi-channel OCT can be used to measure multiple distances that couldbe used to investigate in real time strain, force, temperature, and thelike. When a force is applied to a spring or an elastic material, threechannels OCT can monitor in real time three distances that could be usedto measure the force directions and its amplitude. If the distancechanges as a result of a thermal expansion material, one channel OCTcould be used to measure temperature in real time.

BRIEF SUMMARY OF THE INVENTION

In various applications such as OCT (Optical Coherence Tomography)imaging for imaging biological tissue or the like, it is desirable toprovide balanced detection in the signal interrogation process,preferably at high efficiency and low cost.

In accordance with an aspect of the present invention, a signalinterrogation system comprises an optical coupler to split input laserbeam into a first laser beam as a power reference and a second laserbeam, the optical coupler being coupled to a first path for the firstlaser beam and a second path for the second laser beam; an opticalcirculator disposed in the second path; a bi-directional optical switchhaving on one side a single channel end and on another side multiplechannel ends with multiple switchable channels, the bi-directionaloptical switch being disposed in the second path, with the singlechannel end oriented toward the optical circulator; a plurality ofoptical fibers coupled to the multiple channel ends of thebi-directional optical switch, wherein the bi-directional optical switchswitches the second laser beam among the plurality of optical fiberscoupled to the multiple switchable channels, the plurality of opticalfibers directing the second laser beam to a target and receivinginterference optical signals based on reflection/scattering lights ofthe second laser beam from the target; an interference optical signalspath coupled to the optical circulator to receive the interferenceoptical signals from the bi-directional switch; and a balanced photodetector to measure a power difference between the interference opticalsignals and the power reference.

In some embodiments, the system further comprises an optical imagingdevice coupled to the plurality of optical fibers to deliver the laserbeam from the optical fibers to illuminate the target, to generatereflection at a fiber end face of a fiber probe of the optical imagingdevice, to receive reflection/scattering lights from the target, and todirect the interference optical signals resulting from interferingbetween the reflection at the fiber end face and thereflection/scattering at the target to the plurality of optical fibers.The fiber probe of the optical imaging device is configured to deliverthe second laser beam to illuminate the target through fiber GRINlenses, the fiber GRIN lenses receiving reflection/scattering lightsfrom the target to interfere with reference lights reflected at thefiber end face. The interference optical signals are generated from thetarget reflection/scattering lights and the reference lights.

In specific embodiments, the system further comprises a variable opticalattenuator disposed in the first path between the optical coupler andthe photo detector. The variable optical attenuator is controlled tomatch power of the first laser beam and the interference optical signalsdirected to the photo detector. The photo detector is tunable to adjusta gain, or the photo detector has a fixed gain. The optical coupler isconfigured to split the input laser beam into about 1% power for thefirst laser beam and about 99% power for the second laser beam; and thebi-directional optical switch has 16 switchable channels.

In accordance with another aspect of the invention, a signalinterrogation system comprises a bi-directional optical switch having onone side a single channel end and on another side multiple channel endswith multiple switchable channels, the bi-directional optical switchbeing disposed in a path of a laser beam, with the single channel endoriented to receive the laser beam, the multiple channel ends directingthe laser beam to illuminate an imaging target and generatereflection/scattering lights from the imaging target and to generatereflection lights from a reference surface, so as to produceinterference optical signals resulting from the reflection lights at thereference surface and the reflection/scattering lights at the imagingtarget; a mechanism coupled to the single channel end of thebi-directional optical switch to receive the interference opticalsignals and to reduce background of laser source intensity profiles fromthe interference optical signals; and a photo detector to measure theinterference optical signals from the mechanism.

In some embodiments, the system further comprises means coupled to themultiple channel ends of the bi-directional optical switch, fordelivering the laser beam to illuminate the imaging target and generatethe reflections/scattering lights from the imaging target and thereference surface, and directing the interference optical signals to themultiple channel ends of the bi-directional optical switch. Themechanism comprises an optical coupler disposed in the path of the laserbeam upstream of the bi-directional optical switch to split a portion ofthe laser beam and direct the portion of the laser beam as a powerreference toward the photo detector; and means for matching powerbetween the power reference and the interference optical signalsdirected to the photo detector. The mechanism comprises an electronichigh-pass filter configured to reduce the background of laser sourceintensity profiles from the interference optical signals. Thebi-directional optical switch has at least 16 switchable channels.

In accordance with another aspect of this invention, a signalinterrogation system comprises a bi-directional optical switch having onone side a single channel end and on another side multiple channel endswith multiple switchable channels, the bi-directional optical switchbeing disposed in a path of a laser beam, with the single channel endoriented to receive the laser beam; a plurality of optical fiberscoupled to the multiple channel ends of the bi-directional opticalswitch, wherein the bi-directional optical switch switches the laserbeam among the plurality of optical fibers coupled to the multipleswitchable channels, the plurality of optical fibers directing the laserbeam to illuminate a target and receiving reflection/scattering lightsfrom the target and receiving reflection lights from a referencesurface, so as to produce interference optical signals between thereflection/scattering lights from the target and the reflection lightsfrom the reference surface; a mechanism to reduce background of lasersource intensity profiles from the interference optical signals; meansfor directing the interference optical signals from the single channelend of the bi-directional switch to the mechanism; and a photo detectordisposed downstream of the mechanism to receive the interference opticalsignals.

In some embodiments, the system further comprises means for deliveringthe laser beam from the optical fibers to illuminate the target togenerate the reflection/scattering lights from the target and to thereference surface to generate the reflection lights from the referencesurface, and directing the interference optical signals to the pluralityof optical fibers.

These and other features and advantages of the present invention willbecome apparent to those of ordinary skill in the art in view of thefollowing detailed description of the specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a 5-channel OCT signalinterrogation system having five detectors, five fiber circulators, anoptical switch, and a signal combiner.

FIG. 2 is a schematic diagram illustrating a signal interrogation systemfor multi-channel OCT using only one photo detector and one fibercirculator according to an embodiment of the present invention.

FIG. 3( a) is a photograph of ablation catheters employed with a 16fibers array for OCT imaging; FIG. 3( b) schematically illustrates afiber probe with a cleaved bare fiber; and FIG. 3( c) schematicallyillustrates a fiber probe with a fiber GRIN lens that is used to form afiber probe array for imaging.

FIG. 4 is a schematic diagram illustrating a signal interrogation systemin which a high-pass filter is used to filter signals upstream of thephoto detector according to another embodiment of the invention.

FIG. 5 shows an intensity plot of two typical interference signals thatwere measured without balanced detection to illustrate the function of ahigh-pass filter for filtering the background laser source intensityprofiles.

FIG. 6( a) is a schematic diagram illustrating one channel OCT fordistance measurement that could be used to measure force or temperature.

FIG. 6( b) is a plot of the signals of three channels OCT showing threedistances between GRIN lenses and reflection surfaces.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part of the disclosure,and in which are shown by way of illustration, and not of limitation,exemplary embodiments by which the invention may be practiced. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Further, it should be noted that while thedetailed description provides various exemplary embodiments, asdescribed below and as illustrated in the drawings, the presentinvention is not limited to the embodiments described and illustratedherein, but can extend to other embodiments, as would be known or aswould become known to those skilled in the art. Reference in thespecification to “one embodiment,” “this embodiment,” or “theseembodiments” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention, and the appearances ofthese phrases in various places in the specification are not necessarilyall referring to the same embodiment. Additionally, in the followingdetailed description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. However,it will be apparent to one of ordinary skill in the art that thesespecific details may not all be needed to practice the presentinvention. In other circumstances, well-known structures, materials,circuits, processes and interfaces have not been described in detail,and/or may be illustrated in block diagram form, so as to notunnecessarily obscure the present invention.

In the following description, relative orientation and placementterminology, such as the terms horizontal, vertical, left, right, topand bottom, is used. It will be appreciated that these terms refer torelative directions and placement in a two dimensional layout withrespect to a given orientation of the layout. For a differentorientation of the layout, different relative orientation and placementterms may be used to describe the same objects or operations.

Exemplary embodiments of the invention, as will be described in greaterdetail below, provide apparatuses and methods for low cost highefficiency signal interrogation technique for multi-channel opticalcoherence tomography.

FIG. 2 is a schematic diagram illustrating a signal interrogation systemfor multi-channel OCT using only one photo detector and one fibercirculator according to an embodiment of the present invention. Such aconfiguration provides multi-channel OCT that is cost-effective andefficient for commercial purposes. In this interrogation process, theoptical switch is only required to act as a bidirectional device. Anadded benefit is that the cost of a bidirectional optical switch can beeven lower than that of a single directional optical switch. For 5 and16 channels, the costs for the signal interrogation are about 20% and 6%of the costs for the current design as shown in FIG. 1. These costs areestimated at the time of drafting this patent application.

As seen in FIG. 2, the signal interrogation system includes a 1/99optical coupler 22, which splits the incident laser beam from awavelength-swept laser source into two beams propagating along twopaths. The first beam (path 24) with ˜1% incident laser power passes avariable optical attenuator (VOA) 26 as a power reference, while thesecond beam (path 25) with ˜99% incident laser power couples to theoptical circulator port-1 and emits at port-2 of the optical circulator28. A polarization controller 27 may be provided upstream of the opticalcirculator 28 to align the input laser polarization to the circulator28. The emitting lights from the optical circulator port-2 connects to a1×16 optical switch 30. The 1×16 optical switch 30 has on one side onesingle channel end (e.g., for single input) and on the opposite sidemultiple channel ends with multiple (16) switchable channels (e.g., formultiple outputs). Other examples of multiple switchable channel opticalswitches usually include 1×2, 1×4, 1×8, 1×2^(N) (N=1, 2, 3, . . . ). Itis a two-way switch that, in one direction, switches the incident lightsamong 16 channels of optical fibers 34 leading to a fiber array (part ofan imaging device 36 such as a catheter), which directs the incidentlights to illuminate the target 40. The reflection or scattering lightsfrom the target 40 will couple back to the same fiber and interfere withthe lights reflected at the fiber end face (see 100 in FIG. 3). Theinterference optical signals propagate back via the channels 34 and the1×16 optical switch 30 in the opposite direction to port-2 of theoptical circulator 28. The optical circulator 28 directs theinterference optical signals from port-2 to port-3 (path 44) as targetinterfered optical signals. The power reference path 24 and the targetinterfered optical signals path 44 both lead to a balanced photodetector 50. A balanced photo detector 50 is a detector that isconfigured to measure the power difference between two inputs. Theoutput of the balanced photo detector 50 is directed to a dataacquisition card 60 (e.g., ATS460) for analog digital convert (ADC) ordata acquisition and processing such as Fast Fourier Transform (FFT) bya computer 70. Meanwhile, the laser source sends a trigger signal via atrigger signal line 51 to trigger the data acquisition card 60 to startcapturing data, and sends an MZI clocking signal via a clocking signalline 52 to re-clock the measured interference signal from equidistanttime spacing into equidistant frequency spacing. The computer 70 employsanother data acquisition card 80 (e.g., NI PC6731) to control the 1×16optical switch 30. The computer 70 is further coupled to the variableoptical attenuator 26 via a control line 90. The computer 70 can alsodisplay the imaging. Note that if the data acquisition card 60 hasenough digital line outputs to control the 1×16 optical switch, thesecond data acquisition card 80 is not required.

FIG. 3( a) is a photograph of ablation catheters employed with a 16fibers array for OCT imaging; FIG. 3( b) schematically illustrates afiber probe with a cleaved bare fiber; and FIG. 3( c) schematicallyillustrates a fiber probe with a fiber GRIN lens. The fiber probes areused to form a fiber probe array for imaging (imaging device 36). Theemitting lights from the fiber array (imaging device 36) illuminate thetarget 40 and the reflection/scattering lights from the target 40re-couple back to the fiber array (imaging device 36). The reflectionlights include object reflection/scattering from the target 40 andreflection from the fiber end face 100 of the imaging device 36 whichserves as reference lights for the target reflection. The referencelights interfere with the target reflection/scattering lights andproduce interference signals for imaging the target 40.

In FIG. 2, the variable optical attenuator 26 in the path 24 iscontrolled by the computer 70 via the control line 90 to perform severalfunctions. It matches the power in the path 24 with the power of theinterference signals in the path 44. The computer-controlled variableoptical attenuator 26 and balanced photo detector 50 can be used toautomatically remove the strong background of low frequency laser powerprofiles from the interference signals to increase the measurementsensitivity.

The photo detector 50 takes in the power reference from the path 24 andthe interference optical signals from the path 44. It converts opticalsignals into analog electronic signals that represent the interferencegenerated by lights reflected from the target 40 and corresponding fiberend face. For example, if the target 40 is a tissue, the frequency ofthe interference signal indicates the depth where the light reflectionor scattering occurs. A fast data acquisition card can be employed toconvert the analog signal into a digital signal. A Fast FourierTransform (FFT) can be used to determine the frequency to reveal thereflection/scattering depth for imaging. It is worth to note that theconverted digital data stream could be in equidistant time spacing,which is required for re-clocking into an equidistant frequency spacing.

FIG. 4 is a schematic diagram illustrating a signal interrogation systemin which a high-pass filter is used to filter signals upstream of thephoto detector according to another embodiment of the invention. Thisembodiment involves another investigation approach that eliminates the1/99 coupler 22, VOA 26, and power reference 24 that are present in theembodiment of FIG. 2. In this investigation technique, only oneinterference optical signal connects to one input of the balanced photodetector, and the detected electronic signal will have a strongbackground of laser source intensity profiles, which would degrade themeasurement sensitivity. An electronic high-pass filter 53 is used tofilter this background to improve the measurement sensitivity. Sincethere is not balanced detection in this investigation technique, asingle input regular photo detector 50′ can be used to measure theinterference optical signals.

FIG. 5 shows an intensity plot of two typical interference signals thatwere measured without balanced detection to illustrate the function of ahigh-pass filter for filtering the background laser source intensityprofiles. The upper curve is measured without a high-pass filter whilethe lower curve is measured with a high pass filter. The high-passfilter removes the strong background of the laser source profile.

Significantly, the present signal interrogation method provides abalanced detection that removes strong signal backgrounds, where a 1/99coupler 22 is used to provide a power reference. This measurementtechnique is referred to as a reference power matched balanceddetection, where a variable optical attenuator 26 may be involved tomatch the power.

In addition, the system employs a fiber array (16 channels 34 in FIG. 2)in conjunction with a single bi-directional optical switch (1×16 opticalswitch 30 in FIG. 2) to simulate the scanning to achieve 2D/3D imaging,so that no optical scanning is required. This is advantageous whenoptical scanning is very difficult or not economical. As discussedabove, the reflection from the fiber end face 100 of the imaging device36 can be used as a reference for the target reflection/scatteringsignals reflected from the target 40, as shown in FIG. 3. The fiber tipof the imaging device 36 may be configured to be angle cleaved or coatedto a desired reflectance (i.e., ˜1% in air). In order to collimate theexit beam, a gradient-index (GRIN) fiber lens 110 can be spliced ontothe fiber probe.

Furthermore, the use of a single bi-directional 1×16 optical switch 30enables the use of a single interference signals path 44 to be used todirect the interference optical signals from the optical circulator 28to a single photo detector 50. This eliminates the cost of additionalphoto detectors, fiber circulators, and signal combiner, and allows thesystem to be made economically and compactly.

For distance measurement, the interferometer is shown in FIG. 4, theintensity of the interfering signal for one channel is given as

$\begin{matrix}{{I = {{r_{0} + {r_{z}{\mathbb{e}}^{j\frac{{\pi \cdot 4}z}{\lambda_{0} + {{{\Delta\lambda}\sin}{({2\pi\; f_{sweep}t})}}}}}}}^{2}},} & (1)\end{matrix}$where r₀ is the amplitude reflectance at the fiber end face and r_(z) isthe amplitude reflectance at z depth of a target imaging object or thegap between fiber and reflection surface as shown in FIG. 6. FIG. 6( a)is a schematic diagram illustrating one channel OCT for distancemeasurement that could be used to measure force or temperature. FIG. 6(b) is a plot of the signals of three channels OCT showing threedistances between GRIN lenses and reflection surfaces. λ₀ is the centralwavelength and Δλ is the wavelength sweeping range. f_(sweep) is thewavelength sweeping rate and Δλ_(fwhm) is the laser instantaneouslinewidth. For simplicity, a top-hat spectral profile

${f({\delta\lambda})} = \{ \begin{matrix}1 & {{{\delta\lambda}} \leq {{\Delta\lambda}_{fwhm}/2}} \\0 & {{{\delta\lambda}} > {{\Delta\lambda}_{fwhm}/2}}\end{matrix} $is used. Eq. (1) can be simplified to Eq. (2), and the interferingsignal is expressed as

$\begin{matrix}{{{ I \sim 2}r_{o}r_{z}{\cos\lbrack \frac{4{\pi \cdot z}}{\lambda_{0} + {{\Delta\lambda}\;{\sin( {2\pi\; f_{sweep}t} )}}} \rbrack}},} & (2)\end{matrix}$where the DC components r₀ ²+r_(z) ² are ignored.

When a Fast Fourier Transform (FFT) is applied to Eq. (2), the Fourierfrequency F is directly proportional to the depth z. Note that are-clocking operation to achieve an equidistant spacing in frequency isrequired for the data stream when it is captured in equidistant timespacing.

$\begin{matrix}{{z = {\frac{\lambda_{0}^{2}}{2{\Delta\lambda}} \cdot \frac{F}{f_{sweep}}}},} & (3)\end{matrix}$

When the spatial displacements result from the applied force and theapplied force is directly proportional to the displacements in a linearsystem, the displacements can be expressed as

$\begin{matrix}{\begin{bmatrix}{\Delta\; x_{1}} \\{\Delta\; x_{2}} \\{\Delta\; x_{3}}\end{bmatrix} = {\begin{bmatrix}A_{1x} & A_{1y} & A_{1z} \\A_{3x} & A_{2y} & A_{2z} \\A_{3x} & A_{3y} & A_{3z}\end{bmatrix}\begin{bmatrix}F_{x} \\F_{y} \\F_{z}\end{bmatrix}}} & (4)\end{matrix}$where λ_(ij), i=1, 2, 3, j=x, y, z are the nine coefficients related tothe mechanical structures and material strengths that can be determinedby experiments. λx_(i) i=1, 2, 3 indicate the three displacements thatcan be calculated by Eq. (3). Rearranging Eq. (4), the three forcecomponents are obtained as

$\begin{matrix}{{\begin{bmatrix}F_{x} \\F_{y} \\F_{z}\end{bmatrix} = {M^{- 1}\begin{bmatrix}{\Delta\; x_{1}} \\{\Delta\; x_{2}} \\{\Delta\; x_{3}}\end{bmatrix}}},} & (5)\end{matrix}$where

$\begin{matrix}{M = {\begin{bmatrix}A_{1x} & A_{1y} & A_{1z} \\A_{2x} & A_{2y} & A_{2z} \\A_{3x} & A_{3y} & A_{3z}\end{bmatrix}.}} & (6)\end{matrix}$

The force value and the direction angles are obtained as

$\begin{matrix}\{ \begin{matrix}{F = \sqrt{F_{x}^{2} + F_{y}^{2} + F_{z}^{2}}} \\{\alpha = {\cos^{- 1}( {F_{x}/F} )}} \\{\beta = {\cos^{- 1}( {F_{y}/F} )}}\end{matrix}  & (7)\end{matrix}$where F_(x), F_(y) and F_(z) are the three components of the force. F, αand β are the value and the direction angles of the force, respectively.When a computer controlled optical switch makes a scan from channel 1 to3 to acquire displacements, the value and direction angles of theapplied force can be real-time measured.

Three fibers are needed to measure three components of force. Thedirection and amplitude of the force can be calculated from the threecomponents. Note that a calibration is required for the calculation ofthe value of the force and its direction.

When the displacement results from temperature, one channel OCT can beused to measure the temperature

$\begin{matrix}{{{Temperature} = {k_{t} \cdot \frac{\lambda_{0}^{2}}{2{\Delta\lambda}} \cdot \frac{F}{f_{sweep}}}},} & (8)\end{matrix}$where k_(t) is the linear thermal/temperature expansion coefficient ofthe material that is used to convert temperature changes into spatialdisplacement which is captured by the optical fiber associated with thechannel for measuring the temperature. For good sensitivity, asufficiently large thermal/temperature expansion coefficient material isusually used to design a certain structure to achieve a length orspatial displacement due to thermal expansion/contraction. For example,a 100-m long glass fiber that has a thermal expansion coefficient of0.5×10⁻⁶ will expand by 50 μm when the temperature increases by 1degree. Note that the refractive index of the fiber changes as well andthe total optical length increases by about 750 μm. As such, thetemperature change can be determined by measuring the change in thisoptical length of the fiber.

The multi-channel OCT can be used to measure in real time multipledisplacements for sensing, such as force, temperature, and the like,where corresponding couplers are used to convert the force, temperature,and the like into spatial displacements. When a plate is moving orrotating, one-channel or multi-channel OCT can be used to monitor inreal time the gap between the plate surface and the fiber to determinethe range of the wave up and down. For 16-channel OCT, 4 channels couldbe assigned to sensing force and temperature, and the remaining 12channels could be used for imaging.

In the description, numerous details are set forth for purposes ofexplanation in order to provide a thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatnot all of these specific details are required in order to practice thepresent invention. Additionally, while specific embodiments have beenillustrated and described in this specification, those of ordinary skillin the art appreciate that any arrangement that is calculated to achievethe same purpose may be substituted for the specific embodimentsdisclosed. This disclosure is intended to cover any and all adaptationsor variations of the present invention, and it is to be understood thatthe terms used in the following claims should not be construed to limitthe invention to the specific embodiments disclosed in thespecification. Rather, the scope of the invention is to be determinedentirely by the following claims, which are to be construed inaccordance with the established doctrines of claim interpretation, alongwith the full range of equivalents to which such claims are entitled.

What is claimed is:
 1. A signal interrogation system comprising: anoptical coupler to split input laser beam into a first laser beam as apower reference and a second laser beam, the optical coupler beingcoupled to a first path for the first laser beam and a second path forthe second laser beam; an optical circulator disposed in the secondpath; a bi-directional optical switch having on one side a singlechannel end and on another side multiple channel ends with multipleswitchable channels, the bi-directional optical switch being disposed inthe second path, with the single channel end oriented toward the opticalcirculator; a plurality of optical fibers coupled to the multiplechannel ends of the bi-directional optical switch, wherein thebi-directional optical switch switches the second laser beam among theplurality of optical fibers coupled to the multiple switchable channels,the plurality of optical fibers directing the second laser beam to atarget and receiving interference optical signals based onreflection/scattering lights of the second laser beam from the target,the interference optical signals for a given optical fiber of theplurality of optical fibers being produced by coupling back thereflection/scattering lights of the second laser beam from the target tothe given optical fiber and interfering with lights reflected at areference surface for the given optical fiber from the second laserbeam; an interference optical signals path coupled to the opticalcirculator to receive the interference optical signals from thebi-directional optical switch; a balanced photo detector to measure apower difference between the interference optical signals and the powerreference; a variable optical attenuator disposed in the first pathbetween the optical coupler and the photo detector; and a computercoupled to the balanced photo detector to receive the measured powerdifference, and to the variable optical attenuator to control thevariable optical attenuator to match power of the first laser beam asthe power reference and the interference optical signals directed to thephoto detector based on the measured power difference.
 2. The signalinterrogation system of claim 1, further comprising: an optical imagingdevice coupled to the plurality of optical fibers to deliver the laserbeam from the optical fibers to illuminate the target, to generatereflection at a fiber end face of a fiber probe of the optical imagingdevice as the reference surface for the optical fibers, to receivereflection/scattering lights from the target, and to direct theinterference optical signals resulting from interfering between thereflection at the fiber end face and the reflection/scattering at thetarget to the plurality of optical fibers.
 3. The signal interrogationsystem of claim 2, wherein the fiber probe of the optical imaging deviceis configured to deliver the second laser beam to illuminate the targetthrough fiber GRIN lenses, the fiber GRIN lenses receivingreflection/scattering lights from the target to interfere with referencelights reflected at the fiber end face; wherein the interference opticalsignals are generated from the target reflection/scattering lights andthe reference lights.
 4. The signal interrogation system of claim 1,further comprising: a control line coupled to the variable opticalattenuator and the computer.
 5. The signal interrogation system of claim1, wherein the computer controls the variable optical attenuator and thebalanced photo detector to automatically remove background of lowfrequency laser power profiles from the interference optical signals toincrease measurement sensitivity.
 6. The signal interrogation system ofclaim 1, wherein the photo detector is tunable to adjust a gain.
 7. Thesignal interrogation system of claim 1, wherein the photo detector has afixed gain.
 8. The signal interrogation system of claim 1, wherein theoptical coupler is configured to split the input laser beam into about1% power for the first laser beam and about 99% power for the secondlaser beam; and wherein the bi-directional optical switch has 16switchable channels.
 9. The signal interrogation system of claim 1,further comprising: a data acquisition unit to receive data includingthe measured power difference from the balanced photo detector foranalog to digital conversion or data acquisition and processing by thecomputer.
 10. The signal interrogation system of claim 9, furthercomprising: a laser source to deliver the input laser beam, to send atrigger signal to trigger the data acquisition unit to start capturingdata, and to send an MZI clocking signal to the data acquisition unit tore-clock the interference optical signals from equidistant time spacinginto equidistant frequency spacing.
 11. A signal interrogation methodcomprising: splitting, using an optical coupler, an input laser beaminto a first laser beam as a power reference and a second laser beam,the optical coupler being coupled to a first path for the first laserbeam and a second path for the second laser beam; providing an opticalcirculator in the second path; providing in the second path abi-directional optical switch having on one side a single channel endand on another side multiple channel ends with multiple switchablechannels, with the single channel end oriented toward the opticalcirculator; coupling a plurality of optical fibers to the multiplechannel ends of the bi-directional optical switch, the bi-directionaloptical switch switching the second laser beam among the plurality ofoptical fibers coupled to the multiple switchable channels, theplurality of optical fibers directing the second laser beam to a targetand receiving interference optical signals based onreflection/scattering lights of the second laser beam from the target,the interference optical signals for a given optical fiber of theplurality of optical fibers being produced by coupling back thereflection/scattering lights of the second laser beam from the target tothe given optical fiber and interfering with lights reflected at areference surface for the given optical fiber from the second laserbeam; coupling an interference optical signals path to the opticalcirculator to receive the interference optical signals from thebi-directional optical switch; measuring, using a balanced photodetector, a power difference between the interference optical signalsand the power reference; providing a variable optical attenuator in thefirst path between the optical coupler and the photo detector; andcoupling a computer to the balanced photo detector to receive themeasured power difference, and to the variable optical attenuator tocontrol the variable optical attenuator to match power of the firstlaser beam as the power reference and the interference optical signalsdirected to the photo detector based on the measured power difference.12. The signal interrogation method of claim 11, further comprising:coupling an optical imaging device to the plurality of optical fibers todeliver the laser beam from the optical fibers to illuminate the target,to generate reflection at a fiber end face of a fiber probe of theoptical imaging device as the reference surface for the optical fibers,to receive reflection/scattering lights from the target, and to directthe interference optical signals resulting from interfering between thereflection at the fiber end face and the reflection/scattering at thetarget to the plurality of optical fibers.
 13. The signal interrogationmethod of claim 12, further comprising: delivering, using the fiberprobe of the optical imaging device, the second laser beam to illuminatethe target through fiber GRIN lenses, the fiber GRIN lenses receivingreflection/scattering lights from the target to interfere with referencelights reflected at the fiber end face; wherein the interference opticalsignals are generated from the target reflection/scattering lights andthe reference lights.
 14. The signal interrogation method of claim 11,further comprising: coupling a control line to the variable opticalattenuator and the computer.
 15. The signal interrogation method ofclaim 11, further comprising: controlling the variable opticalattenuator and the balanced photo detector to automatically removebackground of low frequency laser power profiles from the interferenceoptical signals to increase measurement sensitivity.
 16. The signalinterrogation method of claim 11, wherein the photo detector is tunableto adjust a gain.
 17. The signal interrogation method of claim 11,wherein the photo detector has a fixed gain.
 18. The signalinterrogation method of claim 11, further comprising: splitting, by theoptical coupler, the input laser beam into about 1% power for the firstlaser beam and about 99% power for the second laser beam.
 19. The signalinterrogation method of claim 11, further comprising: providing a dataacquisition unit to receive data including the measured power differencefrom the balanced photo detector for analog to digital conversion ordata acquisition and processing by the computer.
 20. The signalinterrogation method of claim 19, further comprising: providing a lasersource to deliver the input laser beam, to send a trigger signal totrigger the data acquisition unit to start capturing data, and to sendan MZI clocking signal to the data acquisition card to re-clock theinterference optical signals from equidistant time spacing intoequidistant frequency spacing.